Various materials are used for semiconductor devices. For example, silicon nitride (hereinafter, referred to as SiN) films formed by various chemical vapor deposition techniques (hereinafter, CVD techniques) and silicon (hereinafter, referred to as Si) films formed by various CVD techniques are used. The process for producing a semiconductor element includes an etching step which includes a step of selectively removing a specific material over other material.
As one example, a Si film is often used as a hard mask in a semiconductor production process (Patent Literature 1). Since a hard mask is a material required in the step of making a semiconductor element but is unnecessary in the final semiconductor element, the hard mask needs to be removed after the processing involving the hard mask. Furthermore, since a SiN film may be present underneath as a material necessary for the final semiconductor, a process that selectively removes the Si film but not the SiN film may be required.
The removal methods generally include wet etching and dry etching. Since dry etching requires the steps of formation of a resist film, exposure, development, etching and removal of the resist, wet etching has higher production efficiency. Wet etching, however, requires higher removal selectivity with respect to the material.
As a wet etching composition for removing a Si film, an aqueous solution comprising hydrofluoric acid and concentrated nitric acid is known. In this case, concentrated nitric acid serves as an oxidant (Non-patent Literature 1). Concentrated nitric acid that serves as an oxidant, however, generates nitrogen dioxide. Since nitrogen dioxide with a boiling point of 21° C. is highly volatile and highly reactive, it has problems of causing corrosion of the structural members inside the device and the exhaust line and causing air pollution. Moreover, since nitric acid needs to be used at a high concentration to obtain a composition with the performance of interest, waste liquid containing a large amount of nitrogen has a problem of imposing a large burden on the environment.
As a process free of nitric acid, a technique that uses hydrofluoric acid and potassium permanganate is known (Patent Literature 2). While potassium permanganate is decomposed when heated to 200° C., potassium permanganate itself is not volatile. Since a volatile compound is not generated when potassium permanganate is used as an oxidant, it does not cause corrosion of the structural members inside the device and the exhaust line or does not cause air pollution. Moreover, since no nitrogen is contained, no burden is placed on the environment due to nitrogen. Although use of a mixture of hydrofluoric acid and potassium permanganate increases the etch rate of Si, it also increases the etch rate of SiN and thus selectivity required cannot be obtained.
Furthermore, although Si films are known to be removed with an alkaline aqueous solution (Non-patent Literature 1), the etch rate with an alkaline aqueous solution greatly differs depending on the crystal planes. Since crystal planes of a Si film formed by CVD method are facing various directions depending on places, there is a problem that etching hardly proceeds and thus the Si film cannot be removed at places where hardly etched planes are exposed.